Single node for function, import, type decls

Represent function, import, and type declarations with the same
single-node shape as the legacy CFG.

Author: owen-mc

go/ql/lib/semmle/go/controlflow/ControlFlowGraphImpl.qll

@@ -121,6 +121,8 @@ module CfgImpl {
     AstNode getChild(AstNode n, int index) {
       (
         not n instanceof Go::FuncDef and
+        not n instanceof Go::ImportDecl and
+        not n instanceof Go::TypeDecl and
         not skipCfg(n) and
         result = n.getChild(index)
         or
@@ -496,8 +498,6 @@ module CfgImpl {
       n instanceof Go::SelectStmt
       or
       n instanceof Go::SendStmt
-      or
-      n instanceof Go::FuncDecl
     }
 
     predicate additionalNode(Ast::AstNode n, string tag, NormalSuccessor t) {

go/ql/lib/semmle/go/controlflow/IR.qll

@@ -93,6 +93,9 @@ module IR {
       // "before"/"after" leaf node for its declaration), which hosts the
       // initialization write (see `InitParameterInstruction`).
       this.isBefore(any(FuncDef fd).getParameter(_).getDeclaration())
+      or
+      // Function declarations are represented by their merged leaf node.
+      this.isBefore(any(FuncDecl fd))
     }
 
     /** Holds if this instruction reads the value of variable or constant `v`. */
@@ -847,7 +850,7 @@ module IR {
   class DeclareFunctionInstruction extends Instruction {
     FuncDecl fd;
 
-    DeclareFunctionInstruction() { this.isIn(fd) }
+    DeclareFunctionInstruction() { this.isBefore(fd) }
 
     override Type getResultType() { result = fd.getType() }
   }